The present invention relates to dielectric materials formed from polymers having a high dielectric permittivity.
A dielectric material often used for manufacturing capacitors is vinylidene polyfluoride (PVF.sub.2) which has interesting dielectric properties. It may be obtained in the form of thin films. However, because of technological difficulties, it is difficult to obtain films having a thickness less than 5 .mu.m. This imposes limitations on the voluminal capacity of PVF.sub.2 capacitors since this capacity is inversely proportional to the square of the thickness of the dielectric. Efforts have then been made, using different treatments, to improve the values of the dielectric permittivity of PVF.sub.2 for temperature frequency ranges as extensive as possible. It is also desirable for the optimum temperature range in use to be close to the ambient temperature.
Mechanical uniaxial or biaxial stretching increases the value of the permittivity or dielectric constant by about 20%. The relative dielectric constant then passes for PVF.sub.2 from 10 to 12. Stretching induced by rolling allows values between 13 and 15 to be obtained.
Another solution consists in mixing the PVF.sub.2 with ferroelectric powders having high dielectric constants (between 1000 and 2000). This technique, appearing as a simple and efficient means for increasing the value of the constant .epsilon.r of the dielectric up to about 40 or 50, has the major drawback of definitely preventing films from being obtained of a thickness less than 50 .mu.m.
Another possiblity consists in using copolymers rather than homopolymers, for example vinylidine fluoride and trifluoroethylene copolymers, written P(VF.sub.2 --TrFE) or else P(VF.sub.2 --VF.sub.3). This copolymer has the particularity of presenting the ferroelectric type phase transition at a temperature higher than the ambient temperature and which is a function of the ratio VF.sub.2 /VF.sub.3. Nevertheless the temperature range in which the dielectric constant is high (.epsilon.r between 40 and 50) is limited.
More recently we have seen the appearance of terpolymers, for example the terpolymer of vinylidene fluoride, trifluoroethylene and hexafluoropropene written P(VF.sub.2 --TrFE--HFP) or the terpolymer of vinylidene fluoride, trifluorethylene and trifluorochloroethylene written P(VF.sub.2 --TrFE--TrFC1E). These terpolymers again present a ferroelectric character but the transitions at the Curie temperature are diffused. The result is that the temperature range for which the material has a high dielectric constant is enlarged with respect to that of the copolymers, but still insufficiently.
To overcome these drawbacks, the invention provides ferroelectric polymer alloys at different Curie temperatures, these alloys optimizing the dielectric properties over an extensive temperature-frequency range. These alloys may be based on copolymers, on a homopolymer and one or more copolymers or else based on terpolymers.